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Generation Methods
void→IEnumerable
Method Description
Empty Creates an empty sequence
Repeat Creates a sequence of repeating elements
Range Creates a sequence of integers
Empty, Repeat, and Range are static (nonextension) methods that manufacture simple
local sequences.
Empty
Empty manufactures an empty sequence and requires just a type argument:
foreach (string s in Enumerable.Empty())
Console.Write (s); //
In conjunction with the ?? operator, Empty does the reverse of DefaultIfEmpty. For
example, suppose we have a jagged array of integers, and we want to get all the
integers into a single flat list. The following SelectMany query fails if any of the inner
arrays is null:
int[][] numbers =
{
new int[] { 1, 2, 3 },
new int[] { 4, 5, 6 },
null // this null makes the query below fail.
};
IEnumerable flat = numbers.SelectMany (innerArray => innerArray);
Empty in conjunction with ?? fixes the problem:
IEnumerable flat = numbers
.SelectMany (innerArray => innerArray ?? Enumerable.Empty ());
foreach (int i in flat)
Console.Write (i + " "); // 1 2 3 4 5 6
Range and Repeat
Range and Repeat work only with integers. Range accepts a starting index and count:
foreach (int i in Enumerable.Range (5, 5))
Console.Write (i + " "); // 5 6 7 8 9
Repeat accepts the number to repeat, and the number of iterations:
foreach (int i in Enumerable.Repeat (5, 3))
Console.Write (i + " "); // 5 5 5
412 | Chapter 9: LINQ Operators